Structural requirements for the formation of the B 2 El VI N 2 heterocycles

The reactions of the bis(diorganoboryl) chalcogenides (R 2 B) 2 El VI [R = Et, El VI = O ( 9 ); R 2 = 1,5‐C 8 H 14 ( 1 , El VI = O), ( 2 , El VI = S)] with 3‐methyl‐, 3,5‐dimethyl‐, 3‐methyl‐5‐phenyl‐, 3,5‐diphenyl‐, and 3,5‐di‐ tert ‐butylpyrazole [ mPz, m 2 Pz, mpPz, p 2 Pz , and (tb) 2 Pz , respectively] have been investigated. mPz reacts with 2 to form the heterocycle m5 , which due to the interaction of the methyl substituent with the carbon skeleton of 2 is unstable and readily rearranges to the novel heterocycle 7 (X‐ray analysis) when dissolved in toluene at room temperature. In contrast the analogues m4 (El = O), formed from mPz and 1 , is stable even when heated at 100°C. The reaction of 1 with m 2 Pz and mpPz does not produce stable heterocycles of the type 4 , instead the 1:1 adducts m 2 Pz ‐ 1 and mpPz ‐ 1 are formed in which the pyrazole fluctuates between the two boron atoms of 1 . The diboryl oxide 1 showed only weak interactions with mpPz and none with p 2 Pz and (tb) 2 Pz . With 9 the pyrazoles m 2 Pz, mpPz , and p 2 Pz from readily the corresponding heterocycles 10 , however no reaction was observed with (tb) 2 Pz ( 1 H‐, 11 B‐, 13 C‐NMR).